This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Terra-cotta is the name applied to a material produced by burning any fine variety of clay which will vitrify upon the surface at a comparatively low temperature. The many advantages possessed by terra-cotta have caused it to become one of the most important building materials of the present day, and so, indeed, is it likely to remain unless the art of its manufacture is lost.
Terra-cotta lends itself well to all kinds of plain or decorated work both for the external and internal parts of a building.
As a substitute for stone it possesses many distinct advantages, chiefly in its weathering properties, there being many examples of terra-cotta which, in spite of many centuries of exposure to the atmosphere, are practically as perfect now as on the day they left the kiln, while the finest of stones similarly exposed have crumbled away.
Terra-cotta work is cheaper than stonework, particularly when features are frequently repeated in a design, and so only require one mould for a number of pieces. Moreover, terra-cotta is strong, and easily moulded or modelled to all shapes, while it can be made in numerous beautiful colours, and it is clean and sanitary, the two last properties rendering it particularly useful for smoky towns.
For building purposes terra-cotta is made into hollow blocks with shells about 2 inches thick, these shells being stiffened when the blocks are large by means of webs to prevent them from warping or twisting when fired in the kiln.
The size of the blocks should be limited to about 3 1/2 cubic feet, but the smaller they are the less likely are they to warp in the firing.
To the manufacturer there are but three descriptions of terra-cotta, namely - Plain, Moulded, and Enriched. Plain Terra-Cotta. - This is the name given to terra-cotta work which has plain faces only; that is to say, work which resembles plain ashlar work.
When building terra-cotta walls the hollow spaces within the blocks should be filled with concrete, very great care being taken that the cement or lime used for this purpose may not be hot, so as to expand on setting and burst the shell.
It is a mistake to use terra-cotta as a veneer only, as its strength in compression is very considerable, far exceeding that of Portland stone.
Walls faced with terra-cotta backed with brickwork are built in a similar manner to brick ashlar, the blocks being made to suit the dimensions of the particular brick employed, in order that a proper bond may be obtained. The blocks should be arranged to bond properly with one another, an arrangement similar to Flemish bond being satisfactory both in strength and appearance.
Particular care must be taken in making mouldings in terra-cotta in order that the blocks may be as straight as possible, as the smallest irregularities become very apparent when set in the work in continuous lines, such as cornices and string courses. If enrichments be added, however, irregularities become less noticeable, and as they scarcely add to the cost there is every reason for their employment.
When features of considerable projection are to be formed of terracotta the cavities in the projecting blocks are left unfilled, advantage being taken of their lightness to obviate the necessity of having counterbalancing loads to keep them from overturning. Fig. 196A shows how a cornice would be built in terracotta.
The lower blocks are filled in with concrete - lime concrete for preference, so as to give sufficient weight within the thickness of the wall to enable the upper projecting blocks to be stably supported. These are kept small by building them up as shown while they are left unfilled, and so are light to carry. The upper weathered surface is formed of a slab in preference to a block.
The tailed-in blocks can be further supported by means of angle irons resting upon them and running along the whole length of the cornice, these latter being anchored down at intervals by means of bolts passing down and terminating in iron plates built into the body of the wall. The blocks forming the balustrade, if there be one, are either filled up with concrete so that the weight may add to the stability of the whole, or supported by a light framework of iron. The method of fastening every part of the terra-cotta work should always be determined before the terra-cotta is made, so that holes may be made therein for the insertion of the fastening if these are to be of metal, but it is better to avoid them and trust to the weight of concrete filling to counterbalance any projections.
The upper surfaces of projecting mouldings should be weathered, i.e. sloped so as to throw off rain water, and if large they should be built up in pieces, the upper pieces overlapping the corner pieces as shown. The vertical joints should be made to overlap by moulding small projecting strips of clay upon one end of each piece to keep the water out of the joint.
This term is applied to work in which modelled ornament occurs, such as foliage or figure modelling. If a feature is decorated with undercut ornament it is usually modelled by hand, but if any enrichment repeats throughout a design a mould is made for it, and the undercut portions are worked upon the moulded clay.
When openings have to be spanned in terra-cotta by means of an arch of considerable rise the blocks are formed into wedge-shaped voussoirs; but if the arch is flat the limited size of the block renders the use of a lintel out of the question, so that a flat arch must be formed, the voussoirs having their bed joints stepped as in Fig. 197 or joggled as in Fig. 198, to prevent them from sliding out of place.
Sills are sometimes formed in one piece, in which case only the ends should be bedded, otherwise the weight brought upon them by the jambs may cause them to crack at the middle. If the sill is built of a number of pieces the joints should be lapped to keep out the water, and the upper edge should be turned up and let into a groove in the under side of the wood sill, so as to form a watertight joint as shown in Fig. 197.
In setting terra-cotta blocks care should be taken to see that each block is fixed in the exact position indicated in the key drawing supplied by the manufacturer, and when terra-cotta is used in connection with brickwork the latter should be set out with very great accuracy, so that the terra-cotta may fit into its place exactly.
Before filling the blocks with concrete they should be thoroughly soaked in water, to ensure the proper adhesion of the two materials.
The Mortar Joints are usually about 1/4 inch wide, but the width depends upon the regularity of the blocks, or upon the size of the blocks, which usually amounts to the same thing. A weather-struck joint is usually employed, as it prevents the accumulation of water therein.
The joints are sometimes raked out to a depth of about 1 inch, and caulked for about 1 inch with oakum and finally pointed, so that should the mortar at the outer surface become disintegrated and so form a lodgment for water this cannot penetrate the joint.
Faience is a form of glazed stoneware which is used considerably for building purposes. It can be made in nearly every imaginable colour, so that a most charming effect can be produced. This material is particularly useful for use in smoky towns, where its colour adds a pleasing relief to the grimy surroundings, while its property of reflecting light has caused it to be used considerably for the facings of courtyards and open areas. Its application to buildings is similar to terra-cotta, save that it is usually made in larger pieces.